An aluminum substrate has been widely used as a substrate for a magnetic recording medium such as a magnetic disk. As the magnetic disks are being produced in smaller sizes and smaller thickness but must record data at a higher density, the aluminum substrate is now being gradually replaced by a glass substrate having superior surface flatness and substrate strength. As glass substrates for magnetic recording medium, there have been used chemically strengthening glass substrates for enhancing the substrate strength and crystallized glass substrates featuring increased substrate strength based on the crystallization.
Accompanying the trend toward high-density recording, further, the magnetic head, is being changed from a thin-film head to a magneto-resistive head (MR head) and to a giant magneto-resistive head (GMR head). It is therefore expected that reproducing the contents the magnetic recording medium of the glass substrate by using a magneto-resistive heads will become standard in the future.
Thus, a variety of improvements have been made to the magnetic disk for high-density recording. Advances in the magnetic disk, however, are also accompanied by new problems concerning the glass substrate for the magnetic recording medium. One of the problems is to cleanse the surfaces of the glass substrate. That is, adhesion of a foreign matter on the surfaces of the glass substrate could become a cause of defects in the thin film formed on the surfaces of the glass substrate or a cause of protuberances formed on the surfaces of the thin film. Further, in reproducing the contents of the magnetic recording medium of the glass substrate by using a magneto-resistive head, if the flying height (floating height) of the head is reduced to increase the recording density, there may often occur erroneous reproducing operations or a phenomenon that the reproduction is not accomplished. The cause is that the protuberances formed on the surface of the magnetic disk, due to particles on the glass substrate, turn into thermal asperity, generating heat at the magneto-resistive head, varying the resistance of the head, and adversely affecting the electromagnetic conversion.
A principal cause of foreign matter on the surface of the glass substrate for the magnetic recording medium described above stems from the end surface of the glass substrate not being smooth and, hence, the end surface abrades the wall surface of the resin casing, whereby resin or glass particles formed by the abrasion, as well as other particles trapped on the inner peripheral end surface and the outer peripheral end surface of the glass substrate, adhere to the surfaces.
Patent document 1 (JP-A-11-221742) discloses a polishing method wherein a disk-like glass substrate (substrate for a recording medium) having a circular hole at the central portion is immersed in a polishing solution containing free grains, and the inner peripheral end surfaces and/or the outer peripheral end surfaces of the glass substrate are polished by being brought into rotational contact with a polishing brush or a polishing pad by using the polishing solution containing the free grains.
However, use of the polishing brush together with the slurry is not capable of favorably polishing the substrate for a recording medium such as a small hard disk (e.g., a doughnut-shaped substrate having an inner diameter of not larger than 7 mm). Further, the conventional slurry contains cerium oxide and the like which cannot be easily removed by washing. Besides, the residual cerium oxide may make it difficult to obtain satisfactory reliability in performance in the applications where a highly reliable performance is required, such as in the use of a car-mounted hard disk (HD) and the like. [Patent document 1] Japanese Unexamined Patent Publication (Kokai) No. 11-221742